Unidirectional Photo-induced Charge Separation and Thermal Charge Recombination of Cofacially Aligned Donor-Acceptor System Probed by Ultrafast Visible-Pump/Mid-IR-Probe Spectroscopy

[摘要] A new -stacked donor-acceptor (D-A) system, [Ru(1-([2,2`-bipyridine]-6-yl-methyl)-3-(2-cyclohexa-2`,5`- diene-1,4-dionyl)-1H-imidazole)(2,2`:6`,2``-terpyridine)][PF6]2 (ImQ_T), has been synthesized and characterized. Similar to its precedent, [Ru(6-(2-cyclohexa-2`,5`-diene-1,4-dione)-2,2`:6`,2``-terpyridine)(2,2`:6`,2``- terpyridine)][PF6]2 (TQ_T), this system has a cofacial alignment of terpyridine (tpy) ligand and quinonyl (Q) group, which facilitates an electron transfer through -stacked manifold. Despite the presence of lowest-energy charge transfer transition from the Ru-based-HOMO-to-Q-based-LUMO (MQCT) predicted by theoretical calculations by using time-dependent density functional theory (TD-DFT), the experimental steady-state absorption spectrum does not exhibit such a band. The selective excitation to the Ru-based occupied orbitalsto- tpy-based virtual orbital MLCT state was thus possible, from which charge separation (CS) reaction occurred. The photo-induced CS and thermal charge recombination (CR) reactions were probed by using ultrafast visible-pump/mid-IR-probe (TrIR) spectroscopic method. Analysis of decay kinetics of Q and Q state CO stretching modes as well as aromatic C=C stretching mode of tpy ligand gave time constants of <1 ps for CS, 1-3 ps for CR, and 10-20 ps for vibrational cooling processes. The electron transfer pathway was revealed to be Ru-tpy-Q rather than Ru-bpy-imidazol-Q.

[学科分类] 化学（综合）

[关键词] Ru;NHC;Electron transfer;Charge separation;Time-resolved IR spectroscopy